A support sheet of known photographic paper generally has a multi-layered structure in which a polyolefin (usually, polyethylene) resin layer containing titanium dioxide (often referred to as "titanium dioxide-containing polyolefin resin layer") is provided on one surface of paper sheet, and another polyolefin resin layer containing no titanium dioxide is provided on the other surface. On the surface provided with the titanium dioxide-containing polyolefin resin layer, a photographic emulsion layer is formed to prepare a photographic paper. Since the resin-coated support sheet having such structure gives an image of high quality, it is also used as a support sheet of image recording paper to be used in combination with various printers such as a sublimation printer and a thermal transfer printer.
The polyolefin resin layer containing titanium dioxide (titanium dioxide-containing polyolefin resin layer) is usually formed in the following manner. Titanium dioxide and a portion of a polyolefin resin are mixed and kneaded under heating by means of a mixer or kneader (e.g., Banbury mixer) so that the resulting mixture can contain titanium dioxide at a high concentration. The mixture was then cooled, dried and treated to give polyolefin resin pellets (that is, master pellets) containing titanium dioxide in a high concentration. Thus prepared master pellets and another portion of a polyolefin resin (which generally is the same as the resin in the master pellets) are introduced into an extruder so that the resulting mixture can contain titanium dioxide in a predetermined concentration, and mixed and kneaded under heating. The kneaded mixture is supplied to a slit die (i.e., a die having a slit for extrusion of a resinous mixture), and extruded through the slit to give a resin film. Onto a paper sheet, the resin film is coated and cooled to give a support sheet having a titanium dioxide-containing polyolefin resin layer on its surface.
The process described above is generally referred to as "master batch method", and is widely used for preparing a resin-coated support sheet of photographic paper because the concentration of titanium dioxide can be easily controlled. On thus prepared resin-coated support sheet, a photographic emulsion layer is placed to give photographic paper.
A photographic image formed on the photographic paper is desired to have improved sharpness, and hence various studies on the support sheet have been reported. For example, Japanese Patent Provisional Publication No. 62(1987)-125345 reports that the image sharpness can be improved by increasing the concentration of titanium dioxide in the polyolefin resin layer of the resin-coated support sheet of photographic paper (its concentration is preferred to be in the range of 9 to 20 wt. %). This publication further reports that a high concentration of titanium dioxide often causes microgrit and/or die lip stain to decrease the quality of products, and further it lowers the productivity of the resin-coated support sheet. The word "microgrit" here means fine foreign material appearing on the surface of the titanium dioxide-containing polyolefin resin layer, and the word "die lip stain" means needle-like piece or stain produced at the end of the die lip after the kneaded mixture is extruded from the slit die.
In the description of the publication, therefore, titanium dioxide is used in a relatively low concentration so as not to cause microgrit and die lip stain, and the resin is used in the form of powder to give photographic paper giving improved sharpness. Since the resin is usually prepared in the form of pellets, the pellets must be beforehand pulverized to give the powder for the use in the process of the above-mentioned publication. The step for pulverizing the pellets, however, extremely lowers the productivity, and further causes unfavorable phenomena such as production of die lip stain if titanium dioxide is used in a relatively high concentration.
Japanese Patent Provisional Publication No. 9(1997)-50093 describes the following. In the case where a polyolefin resin containing titanium dioxide in a concentration of not less than 20 wt. % is extruded from a die lip at a temperature of 290 to 350.degree. C. to give a resin layer, cracks and lines (carved by wastes attached to the end of the die lip) are liable to appear on the surface of the formed layer. The publication describes that such troubles are induced by water attached to (or combined with) the surface of the titanium dioxide particles. Therefore, in order to avoid the troubles, the publication proposes that the titanium dioxide be beforehand treated with a silane coupling agent. However, since the treatment with a silane coupling agent requires complicated process, it is very disadvantageous from the viewpoint of productivity to use titanium dioxide having a surface beforehand subjected to hydrophobic treatment such as the treatment using a silane coupling agent.